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Turbulent heating of the corona
and solar wind: the heliospheric
dark energy problem
Stuart D. Bale
Physics Department and Space Sciences Lab
University of California, Berkeley, USA
IAP 2012 seminar, PSFC/MIT, January 11, 2012
The Sun
A boring, middle-aged star
G type, population 1, ‘yellow dwarf’
Photospheric blackbody ~5000-6000K
Sunspots and ‘active regions’
Impulsive Solar Activity
- 
- 
- 
- 
- 
‘Carrington Event – September 1-2, 1859
Brilliant, intense aurora borealis (18 hrs later)
Disruption of telegraph services
Once per 500 years (ice cores)
Solar-terrestrial connection
-  Interplanetary space is not empty!
Comet tails
Comets have two tails
-  A ‘Dust tail’ is diffuse and follows the comet’s orbit (Keplerian)
-  A ‘Gas tail’ which points away from the Sun (Biermann)
gas
dust
Comet tails - 2010
The solar corona
1919 eclipse photo, Sobral
1571, Caron
The solar corona
Coronal structure often resembles magnetic lines of force
Eclipse observations show the ‘solar corona’
Thomson-scattered white light
The corona is very hot and magnetized
Hydro Scale height (H ~ kT/mg) not consistent with simple hydrostatic
equilibrium
Using 6000 degrees C as a temperature, if the atmosphere is hydrogen then H
= 175 km (110 miles) – solar radius is much larger
Instead, from the eclipses the scale height is clearly comparable to the radius
of the Sun, or H = 695,500 km (430,000 miles)
- So the corona is very hot or we have some new, lighter elements ‘ coronium’
Early spectrographic measurements of
mysterious emission lines helped the
confusion (again ‘coronium’)
Edlén (1942) identified line emission with
highly ionized Fe implying electron
temperatures of T >106 C
The corona is very hot and magnetized
K corona – photospheric light scattered from
electrons – spectrum is washed out by Doppler-shift
F corona – photospheric light scattered from dust,
solar spectrum remains – ‘zodiacal light’
E corona – emission lines from ionized, heavy
elements in the corona – UV-soft x-ray
- H and He are fully ionized – no emission
- Minor ions are partially (and often highly) ionized
- Polarization/splitting of emission lines gives line-ofsight magnetic field (~1 G)
The corona is a tenous, hot magnetized plasma
An important measurement: perpendicular heating
(Cranmer et al., 2008)
A summary (to 1950’s)
-  The solar photosphere is ~6000K and macroscopically
‘homogeneous’ (and β = nkT/(Β2/2µ0) > 1) – a ‘fluid’
-  Impulsive events and flares on the Sun produce activity
in the Earth’s ionosphere – transit time is hours (slow) space is not empty!
-  Comet ‘gas’ tails point away from the Sun – fast flow!
-  The solar corona is tenuous and highly structured –
often organized by magnetic ‘lines of force’
-  The solar corona is very hot (> 106 K) – this temperature
inversion is puzzling (2nd law of thermodynamics)
-  The high coronal temperature and emission lines
suggest that the gas is highly ionized, i.e. a magnetized
collisionless plasma (β << 1)
Parker’s solar wind model
-  Hydrostatic solution
(similar to Bondi
accretion)
-  Predicts a supersonic
atmosphere ‘wind’
-  Similar to ‘de Laval
nozzle’ or a jet engine
-  Requires energy input
at the base. kTph is not
nearly enough!
Requires nonthermal
energy
-  ‘Alfven point’ in
magnetized plasma
determines extent of
corona - corotation
A ‘solar wind’ is accelerated from the corona
Mariner 2 measurements
Parker’s solar wind is confirmed
The solar wind is highly variable
The solar wind is heated continuously
-  Helios spacecraft
measurements from
0.3 – 1 AU
-  Voyager spacecraft
measurements
outward
-  Tp ~ 1/r
-  Free expansion
predicts a much more
rapid decay
-  Requires continuous,
distributed energy
input
Kinetic Physics in the Corona and
Solar Wind
(Marsch et al)
B
B
•  There are very few
collisions in the solar wind
•  Not in thermal equilibrium
•  Large temperature
Relative Frequency
anisotropies – heating is
organized by magnetic
field
•  Different temperatures
•  Relative drifts
THe/TH
(Kasper et al)
electrons
The solar wind is bimodal
Summary #2
-  The corona requires a non-thermal source of heat
-  A sufficiently heated corona will expand super-sonically
and super-Alfvenically to form a ‘solar wind’
-  The expanding solar wind requires additional heating
The large coronal magnetic energy density is a sufficient
energy source! This is our ‘dark energy’. But problems
remain:
1.  How are the magnetic fields created and transported
2.  How is the magnetic energy converted to thermal
energy: magnetic reconnection, shocks, waves and
turbulence
3.  What is the role of ambipolar electric fields?
source of energy
Photospheric motion, granulation, footpoint shuffling
Alfven waves in the corona
NASA Solar Dynamics Observatory (SDO) – Advanced Imaging Assembly (AIA)
High cadence, high resolution 193Å coronal imaging (Vourlidas and Stenborg)
Steady outflows – reconnection? Alfven wave Poynting flux?
Alfven waves
CoMP at NSO
FeXIII at 1074.7 nm
-  ‘Waves’ are faster than sound
-  Abundant in the solar wind
(Belcher and Davis)
-  Propagate along magnetic field
-  Low intensity (in white light)
Hinode (JAXA) CaII measurements
Plasma wave launching
- 
- 
- 
Footpoint ‘shuffling’ generates currents, magnetic fields
Alfven waves propagate upward
Produce a turbulent cascade that terminates in damping
Damping heats the plasma
Turbulent ‘eddies’
evolution
viscous damping
Neutral fluid turbulence
Magnetized turbulent ‘eddies’
evolution
Ambient magnetic field
collisionless damping
Magnetic Plasma Turbulence
Magnetic Plasma Turbulence
Perpendicular cascade
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- Current balance (thermal, photoelectron,
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determine floating voltage
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Perpendicular cascade
(Bale et al., 2005)
Summary #3
-  Alfven waves appear to be generated low in the corona.
-  Magnetized turbulence is measured in the outer
heliosphere – consistent with a perpendicular cascade
-  However, remote-sensing (UVCS) ion temperatures
suggest cyclotron heating and hence significant high
frequency compressive waves.
-  What is the role of reconnection? Ambipolar electric
fields? Shocks?
-  We need radial profiles, we need to get inside of the
Alfven radius (10-15 Rs)
-  We need modern, high-quality in situ measurements in
the inner heliosphere
NASA Solar Probe Plus
-  Launch in 2018
-  Mostly in situ instruments
-  Perihelion at 9.5 Rs –
within the Alfven radius
-  Lots of orbits…
-  NASA ‘Living with a Star’ Mission
-  Recommended by NAS for 30 years
-  Most ambitious NASA ‘Heliophysics’ mission
Solar Probe Plus
4 x Voltage (electric field) sensors
3 x Magnetometers
- 
- 
The ‘FIELDS’ Experiment for Solar Probe Plus
Excellent magnetic and electric fields
Excellent plasma measurements
 
SWEAP (Kasper, SAO+UCB)
 
Solar wind plasma
 
FIELDS (Bale, UC Berkeley)
 
Electric and magnetic fields
 
ISIS (McComas, SwRI)
 
Energetic particles
 
WISPR (Howard, NRL)
 
White light imager
Solar Probe Plus
Requires heroic thermal engineering!
-  TPS ~ 2000C
-  FIELDS antennas ~ 1300C
-  FIELDS magnetometers ~ -100C
Requires some interesting ops
- 
- 
- 
- 
Initial warm up of radiators
Dust environment
Cp/Cg problems
Solar panels and power
Solar Probe Plus
2018 launch
35 Rs initial perihelion
7 x Venus Gravity Assist
(300 km Venus flyby)
9.5 Rs final perihelion
End 2027
Solar Probe Plus
Launch
July 2018
Solar Probe Plus
Advanced Technology
Solar Telescope (ATST)
-  4.2m, off-axis pupil AO
telescope on
Haleakala, Maui
-  Optimized for dynamic
range and low
scattering
-  Will resolve magnetic
(electric?) fields with
~70km resolution at
~1-2 Rs
-  Connection with Solar
Probe Plus
-  Funded by US NSF, first
light in ~2017
Perspective
-  Magnetic fields are likely to provide the missing ‘dark’
energy for coronal heating and solar wind acceleration
-  Magnetized turbulence is likely to generate the required
solar wind continuous heating
-  The plasma physics remains an open question
-  Alfven waves and turbulence
-  Magnetic reconnection
-  Shock waves
-  Ambipolar fields
-  Physics may be similar to collisionless accretion
-  The coming decade will be a ‘golden age’ for coronal
and solar wind physics: STEREO, SDO, IRIS, Solar Orbiter,
Solar Probe Plus, FASR, and ATST